It doesn’t matter much what I think. Because it’s a government derived regulation is your first indication that it makes no sense whatsoever, but Daryl is right. It has everything to do with worries about ricochets etc. The farms here are typically and on average 100 acres each. Some a little bigger, some a little smaller. The laws go back to the days where small high velocity bullets were much more frangible than they are today, and the ubiquitous and extremely common in large quantities army surplus .303 etc was mostly FMJ. High population density farm land and high powered rifles are not a great mix. There is ongoing argument about whether a .270 Winchester is legal to shoot in southern Ontario, but because it is .277”, it does indeed exceed the legal limit of .275”. The law was written to state that “in southern Ontario, it is not permitted to use a caliber of .275” or greater to hunt small game.”. Then in next section it goes on to detail what types of firearms are legal to hunt big game in all the counties in southern Ontario - mostly bow, muzzle loaders and shotgun only seasons with high powered centre-fire rifles mostly excluded.

Long story short, one cannot go about firing centre fire rifles as one pleases in southern Ontario. Need practice with that .500 Nitro Express? Sure, then join a gun club. I don’t like it, but it is what it is, so off to the range I go to piss off all the fudds shooting their .30-30s, and the target guys shooting their target .22 rifles. I shoot what I like and I like big bores. I want to hunt with them and I question the marksmanship capabilities of a man who dusts their rifle off once a year, scrapes the old mold off the lead tip with a fingernail, shoots one or two rounds off a rest to see if it hits a rock on out yonder, and calls it good for deer season. I firmly believe that one MUST practice practice practice in order to achieve and maintain a reasonable level of marksmanship with whatever a man chooses to shoot.